There will be described a background art of the present invention with reference to an example, a hydraulic excavator shown in FIG. 3. The hydraulic excavator includes a crawler-type lower traveling body 1; an upper slewing body 2 mounted on the lower traveling body 1 so as to be able to be slewed around an axis X perpendicular to a ground; and a front attachment 3 attached to the upper slewing body 2 to perform an operation such as excavation. The front attachment 3 includes a boom 4 mounted on the upper slewing body 2 so as to be able to be raised and lowered; an arm 5 mounted on a distal end of the boom 4; a bucket 6 mounted on a distal end of the arm 5; and a plurality of hydraulic cylinders for actuating the boom 4, the arm 5, and the bucket 6, respectively, namely, a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9. On the hydraulic excavator, further mounted are a traveling motor which is a hydraulic motor for causing the lower traveling body 1 to travel and a slewing motor which is a hydraulic motor for slewing the upper slewing body 2.
In this hydraulic excavator, upon lowering the boom 4, there acts potential energy corresponding to a height of the boom 4 on the boom cylinder 7, thus making pressure in hydraulic oil discharged from the boom cylinder 7, namely, return oil, high. From this view point, there has been known a technique for regenerating energy of such a hydraulic actuator as drive force for another hydraulic actuator.
For example, Patent Literature 1 discloses a technique of supplying return oil flowing from a head-side chamber of a boom cylinder to a rod-side circuit of an arm cylinder through a regeneration line, when a combined operation of simultaneously performing a boom lowering operation that is an operation for lowering a boom and an arm pushing operation that is an operation for moving an arm in an arm pushing direction, to thereby increase a speed of the arm pushing action. To a hydraulic circuit having a regeneration function including the technique, provided are: a regeneration valve disposed on a regeneration line and adapted to make an action of opening and closing the regeneration line or to adjust its degree of opening; and a meter-out valve for controlling a flow rate of return oil which flows from a regeneration source (in the foregoing example, a head side of the boom cylinder) to a tank. Respective operations of the regeneration valve and meter-out valve are controlled corresponding to electric signals that are input from a controller as control means.
In the known regeneration-function-provided hydraulic circuit including the technique described in Patent Literature 1, respective actuator circuits for the regeneration source and the regeneration target are always used in only one state where they are connected through a regeneration line, which can involve an inconvenient situation. For example, in the case where signal input from the controller to the regeneration valve or to the meter-out valve becomes abnormal, when a combined operation of simultaneously performing a boom lowering operation and an arm pushing operation, to thereby disable the regeneration valve or the meter-out valve from being controlled, the lowering operation of the boom, which is the regeneration source, cannot be correctly performed.